One thing I would check is the carrying capacities for the 2 chassis. I would also check the towing capacities. I believe that the Transit has a greater capacity than the Promaster, but I may be wrong/that might just be a Winnebago thing.
The other thing is I think that the Promaster is Gas and the Transit is diesel. Not sure that makes a difference to you, but it is a difference. It probably makes a difference on the Generator options as well.
I have been happy with my Fuse which is Transit based for what it is worth.
2017 Winnebago Fuse 23a
The conFUSEd RVer